This invention relates to packaging materials and structures used in microwave cooking, and specifically to microwaveable packaging of food items which require surface browning and/or crisping during cooking.
There has been much interest recently in packaging materials which aid in browning and crisping of food items in a microwave oven. U.S. Pat. No. 4,267,420, Brastad, discloses a food item wrapped with plastic film having a very thin coating thereon. An additional sheet or film of plastic is optionally laminated to the coating for abrasion protection. Other exterior support by more rigid dielectric materials such as paperboard and the like is also disclosed. The coating converts some of the microwave energy into heat which is transmitted directly to the surface portion of the food so that a browning and/or crisping is achieved.
U.S. Pat. No. 4,641,005, Seiferth, discloses a disposable food receptacle for use in microwave cooking, which includes a provision to brown the exterior of the food in the receptacle. A thin layer of an electrically conductive material is incorporated into the receptacle on the food contacting surfaces thereof, so that the conductive layer will become heated by the microwave radiation and will, in turn, brown the exterior of the food in the receptacle. The receptacle includes a smooth surfaced plastic film, as a protective layer, and a support means formed of paper stock material.
U.S. Pat. No. 4,713,510, Quick et al., discloses a microwave ovenable package including a layer of material that will convert a portion of the microwave energy to heat and a layer of paperboard interposed between the energy-converting layer and the food. The energy-converting layer may be carried on a plastic film, and an additional layer of paperboard may be used to sandwich the energy-converting layer and the plastic film between layers of paperboard. For the purpose of providing a more intense heating effect, two energy-converting layers, each on a dielectric substrate, sandwiched together between layers of paperboard, are disclosed.
Laminates of plastic films with thick layers of vacuum deposited metal are also known as packaging materials. For Example, U.S. Pat. No. 4,559,266, Misasa et al., discloses a laminated material comprising (A) a layer composed mainly of polyolefin, (B) a layer composed mainly of, e.g., polyester resin, (C), a metal-vacuum deposited layer, and (D) a layer composed mainly of a transparent thermoplastic resin. This laminated material is used for its superior gas barrier properties and light shielding properties, etc. Such laminates, in order to provide significant gas barrier properties for packaging applications, require deposition of metal (typically aluminum) in sufficient amounts to impart optical densities of greater than 1.0, typically at least 4.0. Such materials are substantially opaque and have light shielding properties, but are not suited for use for microwave heating applications, for which much lower optical densities are required.
Japanese patent application Ser. No. 51 102 072, Mitsubishi, discloses a thermally contractable metal vapor deposited thermoplastic film. A layer of metal, typically 40 millimicrons of aluminum is deposited on the film, which has first been stretched under ordinary conditions. The film is thereafter further stretched in the same direction as previous stretching at 2-25%. After treatment with an anchoring reagent, the film is further stretched. The resulting film has excellent luster and is useful as labels for cans and bottles.
In order to properly brown or crisp foods which are irregular in shape or which have nonplanar surfaces, it is desirable to have a packaging material which is readily conformable to the food. It is also desirable that the material supply enough heat energy to the surface of the food, and provide some degree of microwave shielding for the interior of the food so that the surface can be properly browned or crispened in a short time without the interior becoming overcooked. The present invention provides a film which conforms closely to the shape of a food item by means of shrinking both before and during cooking, provides a high degree of heat to the surface of the food, and provides shielding to the interior portion of the food.